Even though there are increasing demands for applications of the high energy and high pulse repetition rate (PRR) laser systems, however, these system often have difficulties to achieve good pulse to pulse energy stability (PPES) and low time jitter (TJ). Practical applications of the high-energy laser systems with high PRR are therefore hindered due to these technical difficulties. Specifically, the most common technique to achieve high repetition rate with short and high-energy pulse and high peak power laser operation is to implement the laser systems with technologies of the Q-switch regime. Since its invention in 1961 Q-switch technology became a very powerful technical development in modern solid-state lasers. Different techniques have been proposed to apply the Q-switch techniques in the laser cavity. The most important and efficient active Q-switch techniques presently employed by those of ordinary skill in the art are acousto-optical and electro-optical. Each of these techniques uses a related optical phenomenon, which makes it possible to switch the Q factor of the laser resonator. Both active Q-switch techniques perform very well at low PPR laser systems such as PPR rate at a few Hz to a few hundreds of Hz and also providing leak free, i.e. practical absence of pre-lasing and post-lasing oscillations, Q-switch operation. More detail descriptions are described in several technical publications that includes H. Zhang et. Al “Diode-end-pumped electro-optically Q-switched Nd:YLF slab laser”, Appl. Opt., vol. 43 (14), 2004, pp. 2940-2943. K. Du et. al. “Electro-optically Q-switched Nd:YVO4 slab laser with a high repetition rate and a short pulse width”, Opt. Lett., vol. 28 (2), 2003, pp. 87-89. Tiyanagisawa et. al. “11 mJ, 15 Hz single-frequency diode-pumped Q-switched Er, Yb: phosphate glass laser”, Opt. Lett., vol. 26(16), 2001, pp. 1262-1264. Z. Chen et al. “Enhanced Q-switching in double-clad fiber lasers”, Opt. Lett., vol. 23 (6), 1998, pp. 454-456.
However, when there are requirements for the solid-state laser with higher pulse energy of several joules and increasing PRR rates to 10-100 kHz the Q-switch system starts to encounter new problems in achieving highly stable Q-switched laser operation in both energy and time domain. The following just lists a coupled of key issues.                1) The first problem is the pulse-to-pulse energy stability (PPES). The most current state-of-art Q-switch lasers use solid-state laser materials with low energy storage time like Nd:YVO4; Nd:GdVO4 where τ˜0.07-0.1 ms compared to 0.23 ms for Nd:YAG or 1 ms for Yb:YAG systems. They can only achieve pulse-to pulse energy stability along with very small time jitter at PRR up to 10-15 kHz for an expense of reduced Q-switched pulse energy compared to Nd:YAG and Yb:YAG which perform well at only up to ˜5 and 1 kHz respectively. None of these lasers can go up to 100 KHz PRR while preserving high PPES and low TJ.        2) The second problem is related to high energy with Joule level of giant pulse. The acousto-optical modulators are not able to provide necessary contrast between high and low Q factors of the resonator and as a consequence such systems generate pre- and post lasing oscillations. This reduces Q-switch laser PPES and TJ along with decreased efficiency. On the other hand, the electro-optical modulator uses high voltage and that restricts the possibility to scale the pulse repetition rate over 100 Hz. The system performance can not meet current requirements of 1 kHz-100 kHz laser systems with high pulse-to-pulse stability and small timing laser pulse jitter.        
For these reasons, even that the Q switching technology is considering a powerful way of getting high energy operation of lasers, there are still bottlenecks of the technologies of the Q switched lasers. Since its invention in 1961 Q-switch technology became a very powerful technique in modern solid-state lasers development. As it well known the main advantage of the Q-switch technology compared to other pulse creating methods is a possibility to create a giant pulses with incredibly high peak power of up to MW-GW level.
Different techniques have been proposed to Q-switch the laser cavity. The most important and efficient active Q-switch techniques presently employed are acousto-optical and electro-optical. Each of these techniques uses a related optical phenomenon, which makes it possible to switch the Q factor of the laser resonator quickly. Electro-optical modulators, which based on a polarization rotation principle in electro-optically active materials like DKDP or LiNbO3 provide reliable switch of the laser cavity Q-factor but require high voltage applied to the electro-optically active material. The typical voltage range for half wave or quarter wave polarization rotation in electro-optical modulators is 2-10 kV which make it problematic to use such systems in high repetition rate, high energy lasers with PRR over ˜50-70 Hz. From another hand Acousto-Optical modulators which use the phenomenon of the intra-cavity laser beam diffraction on an acoustical grating created in the modulator crystals provide relatively fast, i.e., approximately microseconds, switch of the cavity Q-factor and high repetition rate Q-switching, may not provide enough contrast of the laser cavity Q factor in ON and OFF regime because of the finite diffraction grating efficiency in required order of diffraction and low diffraction angle. However, when the requirements for the solid-state laser pulse energy and PRR go up to the level of several Joules and 10-100 kHz, respectively, Q-switch starts facing new problems in achieving highly stable Q-switched laser operation in both energy and time domain. Each of these issues is further discussed below.
The first problem which arises at high PPR from the fundamental requirement for the pump pulse duration (i.e. timing in laser Q-switch operation) is the pulse to pulse energy stability (PPES)). If the laser material storage time is τ, the necessary pump pulse duration (PPD) to achieve the maximum energy per Q-switch pulse should not exceed 1.1τ before the Q-switch modulator raise the loss in the laser resonator. From another hand reducing the pump pulse duration below τ obviously not only reduces the Q-switched laser pulse energy as it happens in low PRR systems but also creates a condition for poor pulse-to-pulse energy stability and pulse time jitter in high repetition rate regime as a result of the pulse-to-pulse build up time change. The only reliable solution of such problem is to use solid-state laser materials with low energy storage time like Nd:YVO4; Nd:GdVO4 where τ˜0.07-0.1 ms compared to 0.23 ms for Nd:YAG or 1 ms for Yb:YAG systems. This gives the possibility to achieve high pulse-to pulse energy stability along with very small time jitter at PRR up to 10-15 kHz for an expense of reduced Q-switched pulse energy compared to NdYAG and Yb:YAG which perform well at only up to ˜5 and 1 kHz respectively.
The second problem in Q-switched laser development is related to high energy systems when one need to obtain Joule level of giant pulse energy. In such situation acousto-optical modulators are not able to provide necessary contrast between high and low Q factor of the resonator during operation and as a consequence create pre- and post lasing oscillation reducing Q-switch laser efficiency along with decreased PPES and TJ. Electro-optical modulator becomes the only reliable Q-switch approach, which provides practically infinite loss in the laser cavity for the low Q stage of the switching process. However, as it is mentioned above, high voltage used in the electro-optical modulators restricts the possibility to scale the pulse repetition rate over ˜100 Hz which doesn't fit current Army requirements of 1 kHz-100 KHz laser systems with high pulse-to-pulse stability and small timing laser pulse jitter.
Another difficulty encountered by the conventional laser systems is the thermal lens implemented in the gain medium, the thermal lens usually restricts the energy/power scaling and degrade the laser beam quality in traditional solid-state bulk laser systems. Several publications have discussed these issues. G. P. Lee, et al., “980 nm diode pumped Yb doped Q switch fiber laser.” Electron. Lett. 31(21), 1836-1837(1995). G. P. Lee, et al., “Q switched erbium doped fiber laser utilizing a novel large mode area fiber.” Electron. Lett. 33(5), 393-394(1998). V. Dominic, et al., “110 W fiber laser,” Proc. CLEO'99, PD paper CPD26 (1999). B. Desthieux, R. L. Laming, and D. N. Payne, “111 kW Pulse amplification,” Appl. Phys. Lett. 63(5), 585-588 (1993). However, these publications have not provided effective methods to overcome the problem of beam quality degradation caused by the thermal lens implementation.
In addition to the above-discussed problems, there are also difficulties related to the nonlinear effects and SBS mitigation in optical amplifier. For high power optical amplifier, there are several issues relating to its power handling as discussed in many prior publications. Specifically, the publications include D. Taverner, et al., “Generation of high energy pulses using a large mode area erbium doped fiber amplifier,” Proc. CLEO'96, 496-497 (1996). L. Goldberg, et al., “Highly efficienct 4 W Yb doped fiber amplifier pumped by a broad stripe laser diode,” Opt. Lett. 24 (10), 673-675 (1999). A. Galvanauskas, et al., “Compact ultra high power laser system,” SPIE 2377, 117-126 (1995) S. Hofer, et al., “Single frequency master oscillator fiber power amplifier system emitting 20 W of power,” Opt. Lett. 26 (17), 1326-1328 (2001). D. Rafizadeh, et al., “Kilowatt pulses from a single mode erbium doped amplifier,” Electron. Lett. 317-318 (1994). P. A. Champert, et al., “Multiwatt average power continua generation in holey fibers pumped by kilowatt peak power seeded Yb fiber amplifier,” Appl. Phys. Lett. 81(12), 2157-2159 (2002). D. C. Brown and H. J. Hoffman, “Thermal, stress, and thermal-optic effects in high average power double clad silica fiber lasers,” IEEE J. Quantum Electron. 37 (2), 207-217 (2001). Raman Kashyap, Fiber Bragg Gratings, Academic Press, New York, 1999. Jian Liu, Paul Wysocki, M. Andrejco, and Bera Palsdottir, “L-band Erbium doped silica fibers and their applications,” (invited paper), Photonics West'2002, San Jose, Calif. Warren Smith, Modern Optical Engineering, McGraw-Hill (New York), 2000. E. W. Taylor, et al., “Gamma-Ray Induced Effects in Erbium-Doped Fiber Optic Amplifiers”, Proc. SPIE, Vol. 3440, 19-24 July 98, San Diego, Calif. First, power saturation and nonlinear effects are two main contributions. They are all related to the fiber structure. The nonlinear effects such as SBS and stimulated Raman scattering (SRS) are also correlated with power and linewidth of the signal. Secondly, the configuration of the optical amplifier and ASE cleanup has significant effects on its performance, especially at a wavelength of 1064 nm. Thirdly, pump wavelength selection and coupling scheme limits the pump power delivery and amplifier stability and efficiency.
Therefore, a need still exists in the art of fiber laser design and manufacture to provide a new and improved configuration and method to provide a hybrid high power laser to achieve high repetition rate and high pulse energy such that the above-discussed difficulty may be resolved.